Prostate specific antigen (PSA) is widely recognized to be an important marker of prostate cancer, and is currently widely used in general clinical medicine as well as testing and diagnostic applications. As a result, the frequency of detection of patients suspected of having prostate cancer is increasing. PSA exhibits abnormal values not only in prostate cancer but also in benign prostatic hypertrophy (BPH), thus making it difficult to diagnose prostate cancer on the basis of PSA alone. Although a comprehensive diagnosis is made, including rectal examination and diagnostic imaging, the final definitive diagnosis is made on the basis of a biopsy. Biopsy consists of a comprehensive diagnosis made on the basis of such factors as a cancer containment score of biopsied tissue, histopathological findings and PSA-related parameters. However, health care facilities do not have uniform diagnostic criteria for making these diagnoses, and studies are being conducted at numerous facilities for unifying diagnostic criteria even at present. One reason for this is the difficulty in making a histopathological assessment of prostate cancer. More specifically, although diagnosis of prostate tissue is made on the basis of disappearance of basal cells and confirmation of the presence of precancerous and cancerous lesions, since a subjective assessment by a pathologist has a considerable effect on the diagnosis, there is a need for a definitive tumor marker. Although cytoplasm staining marker 34βE12 and cell nucleus staining marker p63 are used in the staining of basal cells, these are not used to investigate the presence of cancer cells directly. Recently, a marker in the form of AMACR (α-methylacyl-CoA-racemase) has been developed that yields stained images specific for cancer cells. There have been numerous reports indicating the effectiveness of this marker for identifying cancer, and it has come to be used in the clinical setting. However, since this marker has unstable stained images and may also indicate a positive result for benign adenocarcinomas, it is not adequate for use as a tumor diagnostic marker by itself. A marker sought after in the clinical setting for diagnosis of prostate cancer is a testing reagent for cancers requiring aggressive treatment, or so-called clinically significant cancers. More specifically, this is a prostate cancer marker that yields stained images specific to advanced cancers only, but does not stain so-called clinically insignificant, latent cancers that are frequently observed in the elderly and do not require treatment. AMACR is unable to distinguish these cancer cells and ends up yielding a positive assessment even for prostate cancer not requiring treatment.